Day 59: Standard-winged nightjar – Macrodipteryx longipennnis
Flying the flag for nightjars
I struggled to find a decent
photo of this splendid nightjar and that fact alone is indicative of nightjar
ecology. Like the owls and frogmouths before them, nightjars are crepuscular or
nocturnal. However they generally live in dry, open country and nest on the
ground so observations are generally made of shapes flying through the twilight
or spotlights picking up flashes of white.
Nightjars are able fliers and
hawk insects from the air, using their facial bristles to aid in capture and
protect the large eyes from flailing insect wings. Measuring nightjar beaks was
intriguing as there was little beak protruding from the skull yet the gape was
as wide as the head, presumably allowing nightjars to effectively become flying
mouths. Their aerial insectivory lends some American species the name 'nighthawk' whilst the name ‘nightjar’ was originally ascribed to the European nightjar (Caprimulgus europaeus) where it referred
both to their nocturnal activity and their remarkable ‘churring’ song which
reverberates around British heathland. The genus Caprimulgus harks back to an old myth that nightjars suckled from
goats (capra=goat, mulgere=to milk).
Although generally cryptic in
colouration, male nightjars are often distinguishable from females by
patches of white on the throat, wings or tail. These are used in courtship
displays; the white reflecting whatever light might be left in the sky. This
African species is notably one of a few nightjars that have modified their
feathers in unusual ways to aid in courtship. The remarkable ‘standards’ of
this species are actually modified primaries which can be erected during
courtship.
Day 60: Edible-nest swiftlet – Aerodramus fuciphagus
Enjoy the bird, not the nest
To look at, this swift is a
typical member of the swift family Apodidae. Swifts are characterised by their
extremely long, scythe-like wings which make them supreme fliers. They are able
to reach tremendous speeds with the white-throated needletail holding the
record for the fastest bird in flapping flight: an eye-watering 112 km/hr (or
more?!). You may have seen our own swifts (Apus
apus) hurtling round the spires and towers of our towns and cities where
they come to breed in the summer. Like the closely related nightjars, swifts
are aerial insectivores however they hunt during the day. They spend the vast
majority of their time on the wing, eating, drinking, mating and sleeping in
mid-flight! In fact, the genus Apus
(from which the names of the family and order are derived) means ‘without
feet’, referring to early misconceptions about swifts.
When swifts do come to land it is
to breed. For us, in the UK, we are used to seeing the convergently evolved
(Passeriformes: Hirundinidae) swallows (Hirundo
rustica) and house martins (Delichon
urbica) building their nests under eaves and beams where they bind together
mud with their saliva. Swifts engage in similar practices but using vegetation over mud. In
the Aerodramus swiftlets, saliva
actually forms the majority of the nesting material and in two species, the
edible-nest swiftlet and the black-nest swiftlet (Aerodramus maximus), saliva is the sole component.
These two species nest in huge
caves dotted around the islands of Southeast Asia. There, they form large colonies, covering the walls with glistening nests. Aerodramus swiftlets navigate around the colonies using
echolocation, an ability that evolved just twice in the birds (the other
incidence being the caprimulgiform oilbird, Steatornis
caripensis).
Such large congregations of birds
make them easy pickings, not least for humans. In China, the nests of edible-nest
and black-nest swiftlets have becoame a delicacy. Served in the infamous
bird's-nest soup, swiftlet nests are believed to bring health benefits and an
increase in libido. This has resulted in the harvesting of some colonies to
near extirpation. In Hong Kong, the centre of swiftlet-nest trade, demand is
high and nests can fetch up to $10,000 per kilo. The creation of artificial
nesting sites has allowed a regular supply of nests, shifting attention away
from the natural sites. As with ivory and tiger claws, it is important that China, and the rest of the world, comes to value our wildlife living rather than dead.
61: Antillean crested hummingbird - Orthorhyncus cristatus
The first of many
I have a hazy memory of the first
time I ever saw a hummingbird. In the year 2000, my sister and I were walking
through hotel gardens on the south coast of Grenada in the Lesser Antilles of
the Caribbean. There, a tiny bird reared up in front of use before promptly
wizzing off in the opposite direction. The bird in question was an Antillean
crested hummingbird, one of over 340 hummingbird species in the second largest
bird family Trochilidae. There is plenty to say about hummingbirds but luckily
I will have a few days to explore them.
Humming birds are generally
placed in the same order as the swifts (Apodiformes). Why this might be is an
interesting question as hummingbirds are very un-swift-like but both groups
share similar wing morphologies. However whereas the swifts use their wings to
zoom through the skies, the hummingbirds have their own specially adapted
flight. Hummingbirds are expert fliers, able to hover and fly
forwards, backwards, up, down and sideways. Using a technique similar to
insects, hummingbirds flex their wings in quick beats generating lift on the
upward stroke in addition to the downward stroke by inverting the curvature of
the wing. With typically between 50 and 80 wing beats per second, the
hummingbirds are able to generate sufficient lift to facilitate their
astounding aerobatics.
To support their flight,
hummingbirds need additional adaptations. They have light, hollow bones with
fusion in the vertebrae to remove unnecessary muscles and ligaments. The feet
are also tiny to reduce weight and drag. By contrast the sternum and pectoral
muscles are enlarged for increased wing power and strong finger bones stabilise
the wing. Perhaps the most important requirement for hummingbirds is the
highest metabolism of any non-insect animals. Their hearts beat up to 20 times
per second and they can breathe 250 times per minute.
Such high energy expenditure requires
near-constant feeding and hummingbirds have made use of the most energy-rich
and readily-available food supply: nectar. They must visit hundreds of
flowers a day in order to drink more than their weight in nectar. Their
metabolism ensures sugar makes it to the muscles as soon as possible but
this means hummingbirds are never far from dying, so constant is their energy
need. Overnight, hummingbirds must shut down, entering torpor where there
metabolism decreases and their body temperature halves. This sleep-like state
allows them to get through the night until the first drink of the morning. Being a hummingbird sounds knackering!
Day 62: Violet Sabrewing - Campylopterus hemileucurus
The purple flash
Violet sabrewings are one of a few hummingbird species that I encountered on my trip to Honduras in 2011, my first Noetropical experience since Grenada. The sabrewing is a stunning bird and the name aptly describes both the striking purple colouration and the unusual wing morphology. I had assumed that 'sabrewing' referred to the overall shape of the wing however closer inspection reveals the broad, flattened central rachis of the first two primary feathers which resemble curved swords. We speculated as to the function of these structures and, being only on the males, the most likely explanation seemed to be for noise generation (by rattling) during competitive displays. Sabrewings are polygynous birds, holding leks where they attract and court multiple females.
Another intriguing difference between males and females is the difference in bill morphology. Despite being smaller, the female has a longer and more curved beak. Again, the explanation for this was not obvious but could be a difference in foraging behaviour between males and females. This is highly unusual in the bird world as most members of the same species tend to have similar diets. It could be the case that males and females specialise on different flowers so as not to compete with each other. Alternatively, (and perhaps more likely), if longer, curvier bills are indicative of greater flower specialisation, perhaps females must specialise more in order to access richer nectar sources for their young. As hummingbirds also eat invertebrates, the same arguments apply for insectivory i.e. males hawking for aerial insects whilst females glean them from foliage.
I have already alluded to the close association of hummingbirds and flowers and what a fascinating relationship it is. Hummingbirds, much like bees or butterflies, are in search of the nutritious nectar provided by flowers. Plants are not altruistically offering a free meal but instead aim to be pollinated with pollen transferred from other flowers. This requires that the pollinator regularly visits flowers of the same species which they may not if they are maximising their opportunities by visiting many species. Flowers can increase exclusivity by making nectar trickier to access. The presents an opportunity for some pollinator species to specialise, gaining a monopoly on particular plant species. These evolutionary pressures often lead to arms races with plant and pollinator become increasingly specialised.
The myriad of plant-hummingbird interactions have lead to diverse beak and flower morphologies. Many flowers will have multiple pollinator species whilst many hummingbirds will pollinate multiple plant species. Some hummingbirds have taken specialisation further. Hermits (Phaethornithinae) have very long, curved bills whilst sicklebills (Eutoxeres spp.) have extremely curved bills for flowers of the family Gesneriaceae. The sword-billed hummingbird (Ensifera ensifera) is the most extreme in specialisation. It uniquely has a bill longer than the rest of its body which it uses to drink from just one species: Passiflore mixta. From the perspective of both plants and pollinators, the extent of specialisation can be seen as a choice of strategies with different species (or even sexes, as above) following different evolutionary trajectories. Generalists benefit from accessing abundant yet low value nectar whilst pollinating haphazardly whereas specialists offer a more reliable service in return for a rich reward.With so many possibilities and niches to fill it is perhaps not surprising that up to 25 species can coexist in the same area.
